Network Working Group A. Melnikov, Ed. Request for Comments: 5092 Isode Ltd. Obsoletes: 2192 C. Newman Updates: 4467 Sun Microsystems Category: Standards Track November 2007
IMAP URL Scheme
Status of This Memo
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
Abstract
IMAP (RFC 3501) is a rich protocol for accessing remote message stores. It provides an ideal mechanism for accessing public mailing list archives as well as private and shared message stores. This document defines a URL scheme for referencing objects on an IMAP server.
The IMAP URL scheme is used to designate IMAP servers, mailboxes, messages, MIME bodies [MIME], and search programs on Internet hosts accessible using the IMAP protocol over TCP.
The IMAP URL follows the common Internet scheme syntax as defined in [URI-GEN]. If :<port> is omitted, the port defaults to 143 (as defined in Section 2.1 of [IMAP4]).
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An absolute IMAP URL takes one of the following forms:
The first form is used to refer to an IMAP server (see Section 4), the second form refers to the contents of a mailbox or a set of messages resulting from a search (see Section 5), and the final form refers to a specific message or message part, and possibly a byte range in that part (see Section 6). If [URLAUTH] extension is supported, then the final form can have the <iurlauth> component (see Section 6.1 for more details).
The <iserver> component common to all types of absolute IMAP URLs has the following syntax expressed in ABNF [ABNF]:
[iuserinfo "@"] host [ ":" port ]
The <iserver> component is the same as "authority" defined in [URI-GEN]. The syntax and uses of the <iuserinfo> ("IMAP userinfo component") are described in detail in Section 3. The syntax of <host> and <port> is described in [URI-GEN].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [KEYWORDS].
This document references many productions from [URI-GEN]. When the document needs to emphasize IMAP URI-specific differences from [URI- GEN] (i.e., for parts of IMAP URIs that have more restricted syntax than generic URIs), it uses a non-terminal i<foo> to define an IMAP- specific version of the non-terminal <foo> from [URI-GEN].
Note that the ABNF syntax shown in Section 11 is normative. Sections 2-6 may use a less formal syntax that does not necessarily match the normative ABNF shown in Section 11. If there are any differences between the syntax shown in Sections 2-6 and 11">Section 11, then the syntax shown in Section 11 must be treated as authoritative. Non- syntax requirements included in Sections 2-6 are, of course, normative.
The "enc-user" part of the "iuserinfo" component, if present, denotes mailbox naming scope. If it is absent, the IMAP URL can only reference mailboxes with globally unique names, i.e., mailboxes with names that don't change depending on the user the client authenticated as to the IMAP server. Note that not all IMAP implementations support globally unique names.
For example, a personal mailbox described by the following URL <imap://michael@example.org/INBOX> is most likely different from a personal mailbox described by <imap://bester@example.org/INBOX>, even though both URLs use the same mailbox name.
The userinfo component (see [URI-GEN]) of an IMAP URI may contain an IMAP user name (a.k.a. authorization identity [SASL], "enc-user") and/or an authentication mechanism. (Note that the "enc-user" also defines a mailbox naming scope as described in Section 3.1). The IMAP user name and the authentication mechanism are used in the "LOGIN" or "AUTHENTICATE" commands after making the connection to the IMAP server.
If no user name and no authentication mechanism are supplied, the client MUST authenticate as anonymous to the server. If the server advertises AUTH=ANONYMOUS IMAP capability, the client MUST use the AUTHENTICATE command with ANONYMOUS [ANONYMOUS] SASL mechanism. If SASL ANONYMOUS is not available, the (case-insensitive) user name "anonymous" is used with the "LOGIN" command and the Internet email address of the end user accessing the resource is supplied as the password. The latter option is given in order to provide for interoperability with deployed servers.
Note that, as described in RFC 3501, the "LOGIN" command MUST NOT be used when the IMAP server advertises the LOGINDISABLED capability.
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An authentication mechanism (as used by the IMAP AUTHENTICATE command) can be expressed by adding ";AUTH=<enc-auth-type>" to the end of the user name in an IMAP URL. When such an <enc-auth-type> is indicated, the client SHOULD request appropriate credentials from that mechanism and use the "AUTHENTICATE" command instead of the "LOGIN" command. If no user name is specified, one MUST be obtained from the mechanism or requested from the user/configuration as appropriate.
The string ";AUTH=*" indicates that the client SHOULD select an appropriate authentication mechanism. (Though the '*' character in this usage is not strictly a delimiter, it is being treated like a sub-delim [URI-GEN] in this instance. It MUST NOT be percent-encoded in this usage, as ";AUTH=%2A" will not match this production.) It MAY use any mechanism listed in the response to the CAPABILITY command (or CAPABILITY response code) or use an out-of-band security service resulting in a PREAUTH connection. If no user name is specified and no appropriate authentication mechanisms are available, the client SHOULD fall back to anonymous login as described above. The behavior prescribed in this section allows a URL that grants read-write access to authorized users and read-only anonymous access to other users.
If a user name is included with no authentication mechanism, then ";AUTH=*" is assumed.
Clients must take care when resolving a URL that requires or requests any sort of authentication, since URLs can easily come from untrusted sources. Supplying authentication credentials to the wrong server may compromise the security of the user's account; therefore, the program resolving the URL should meet at least one of the following criteria in this case:
1) The URL comes from a trusted source, such as a referral server that the client has validated and trusts according to site policy. Note that user entry of the URL may or may not count as a trusted source, depending on the experience level of the user and site policy.
2) Explicit local site policy permits the client to connect to the server in the URL. For example, a company example.com may have a site policy to trust all IMAP server names ending in example.com, whereas such a policy would be unwise for example.edu where random students can set up IMAP servers.
3) The user confirms that connecting to that domain name with the specified credentials and/or mechanism is permitted. For example, when using "LOGIN" or SASL PLAIN with Transport Layer Security
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(TLS), the IMAP URL client presents a dialog box "Is it OK to send your password to server "example.com"? Please be aware the owners of example.com will be able to reuse your password to connect to other servers on your behalf".
4) A mechanism is used that validates the server before passing potentially compromising client credentials. For example, a site has a designated TLS certificate used to certify site-trusted IMAP server certificates, and this has been configured explicitly into the IMAP URL client. Another example is use of a Simple Authentication and Security Layer (SASL) mechanism such as DIGEST-MD5 [DIGEST-MD5], which supports mutual authentication.
5) An authentication mechanism is used that will not reveal any information to the server that could be used to compromise future connections. Examples are SASL ANONYMOUS [ANONYMOUS] or GSSAPI [GSSAPI].
URLs that do not include a user name but include an authentication mechanism (";AUTH=<mech>") must be treated with extra care, since for some <mech>s they are more likely to compromise the user's primary account. A URL containing ";AUTH=*" must also be treated with extra care since it might fall back on a weaker security mechanism. Finally, clients are discouraged from using a plaintext password as a fallback with ";AUTH=*" unless the connection has strong encryption.
A program interpreting IMAP URLs MAY cache open connections to an IMAP server for later reuse. If a URL contains a user name, only connections authenticated as that user may be reused. If a URL does not contain a user name or authentication mechanism, then only an anonymous connection may be reused.
Note that if unsafe or reserved characters such as " " (space) or ";" are present in the user name or authentication mechanism, they MUST be percent-encoded as described in [URI-GEN].
As per Sections 3.1 and 3.2 of this document, the IMAP URI enc-user has two purposes:
1) It provides context for user-specific mailbox paths such as "INBOX" (Section 3.1).
2) It specifies that resolution of the URL requires logging in as that user and limits use of that URL to only that user (Section 3.2).
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An obvious limitation of using the same field for both purposes is that the URL can be resolved only by the mailbox owner. In order to avoid this restriction, implementations should use globally unique mailbox names (see Section 3.1) whenever possible.
Note: There is currently no general way in IMAP of learning a globally unique name for a mailbox. However, by looking at the NAMESPACE [NAMESPACE] command result, it is possible to determine whether or not a mailbox name is globally unique.
The URLAUTH component overrides the second purpose of the enc-user in the IMAP URI and by default permits the URI to be resolved by any user permitted by the <access> identifier. URLAUTH and <access> identifier are described in Section 6.1.
An IMAP URL referring to an IMAP server has the following form:
imap://<iserver>[/]
This URL type is frequently used to describe a location of an IMAP server, both in referrals and in configuration. It may optionally contain the <iuserinfo> component (see Sections 3 and 11). A program interpreting this URL would issue the standard set of commands it uses to present a view of the content of the IMAP server, as visible to the user described by the "enc-user" part of the <iuserinfo> component, if the "enc-user" part is specified.
The <enc-mailbox> field is used as the argument to the IMAP4 "SELECT" or "EXAMINE" command. Note that if unsafe or reserved characters such as " " (space), ";", or "?" are present in <enc-mailbox>, they MUST be percent-encoded as described in [URI-GEN].
The <uidvalidity> field is optional. If it is present, it MUST be the same as the value of IMAP4 UIDVALIDITY response code at the time the URL was created. This MUST be used by the program interpreting the IMAP URL to determine if the URL is stale. If the IMAP URL is stale, then the program should behave as if the corresponding mailbox doesn't exist.
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Note that the <uidvalidity> field is a modifier to the <enc-mailbox>, i.e., it is considered a part of the last "component" (as used in [URI-GEN]) of the <enc-mailbox>. This is significant during relative URI resolution.
The "?<enc-search>" field is optional. If it is not present, the program interpreting the URL will present the entire content of the mailbox.
If the "?<enc-search>" field is present, the program interpreting the URL should use the contents of this field as arguments following an IMAP4 SEARCH command. These arguments are likely to contain unsafe characters such as " " (space) (which are likely to be present in the <enc-search>). If unsafe characters are present, they MUST be percent-encoded as described in [URI-GEN].
Note that quoted strings and non-synchronizing literals [LITERAL+] are allowed in the <enc-search> content; however, synchronizing literals are not allowed, as their presence would effectively mean that the agent interpreting IMAP URLs needs to parse an <enc-search> content, find all synchronizing literals, and perform proper command continuation request handling (see Sections 4.3 and 7 of [IMAP4]).
The <enc-mailbox> and [uidvalidity] are as defined in Section 5 above.
If <uidvalidity> is present in this form, it SHOULD be used by the program interpreting the URL to determine if the URL is stale.
The <iuid> refers to an IMAP4 message Unique Identifier (UID), and it SHOULD be used as the <set> argument to the IMAP4 "UID FETCH" command.
The <isection> field is optional. If not present, the URL refers to the entire Internet message as returned by the IMAP command "UID FETCH <uid> BODY.PEEK[]". If present, the URL refers to the object returned by a "UID FETCH <uid> BODY.PEEK[<section>]" command. The type of the object may be determined by using a "UID FETCH <uid> BODYSTRUCTURE" command and locating the appropriate part in the
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resulting BODYSTRUCTURE. Note that unsafe characters in [isection] MUST be percent-encoded as described in [URI-GEN].
The <ipartial> field is optional. If present, it effectively appends "<<partial-range>>" to the end of the UID FETCH BODY.PEEK[<section>] command constructed as described in the previous paragraph. In other words, it allows the client to request a byte range of the message/message part.
The <iurlauth> field is described in detail in Section 6.1.
URLAUTH is a component, appended at the end of a URL, that conveys authorization to access the data addressed by that URL. It contains an authorized access identifier, an authorization mechanism name, and an authorization token. The authorization token is generated from the URL, the authorized access identifier, authorization mechanism name, and a mailbox access key.
Note: This specification only allows for the URLAUTH component in IMAP URLs describing a message or its part.
The mailbox access key is an unpredictable, random string. To ensure unpredictability, the random string with at least 128 bits of entropy is generated by software or hardware (not by the human user).
Each user has a table of mailboxes and an associated mailbox access key for each mailbox. Consequently, the mailbox access key is per- user and per-mailbox. In other words, two users sharing the same mailbox each have a different mailbox access key for that mailbox, and each mailbox accessed by a single user also has a different mailbox access key.
The authorized <access> identifier restricts use of the URLAUTH authorized URL to certain users authorized on the server, as described in Section 6.1.2.
The authorization token is a deterministic string of at least 128 bits that an entity with knowledge of the secret mailbox access key and URL authorization mechanism can use to verify the URL.
A specific message or message part IMAP URL can optionally contain ";EXPIRE=<datetime>" and/or ";URLAUTH=<access>:<mech>:<token>".
When ";EXPIRE=<datetime>" is used, this indicates the latest date and time that the URL is valid. After that date and time, the URL has expired and server implementations MUST reject the URL. If ";EXPIRE=<datetime>" is not used, the URL has no expiration, but can still be revoked using the RESETKEY command [URLAUTH].
The URLAUTH takes the form ";URLAUTH=<access>:<mech>:<token>", and it MUST be at the end of the URL. It is composed of three parts. The <access> portion provides the authorized access identifiers that may constrain the operations and users that are permitted to use this URL. The <mech> portion provides the authorization mechanism used by the IMAP server to generate the authorization token that follows. The <token> portion provides the authorization token, which can be generated using the GENURLAUTH command [URLAUTH].
The "submit+" <access> identifier prefix, followed by a userid, indicates that only a userid authorized as a message submission entity on behalf of the specified userid is permitted to use this URL. The IMAP server does not validate the specified userid but does validate that the IMAP session has an authorization identity that is authorized as a message submission entity. The authorized message submission entity MUST validate the userid prior to contacting the IMAP server.
The "user+" <access> identifier prefix, followed by a userid, indicates that use of this URL is limited to IMAP sessions that are logged in as the specified userid (that is, have authorization identity as that userid).
Note: If a SASL mechanism that provides both authorization and authentication identifiers is used to authenticate to the IMAP server, the "user+" <access> identifier MUST match the
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authorization identifier. If the SASL mechanism can't transport the authorization identifier, the "user+" <access> identifier MUST match the authorization identifier derived from the authentication identifier (see [SASL]).
The "authuser" <access> identifier indicates that use of this URL is limited to authenticated IMAP sessions that are logged in as any non-anonymous user (that is, have authorization identity as a non- anonymous user) of that IMAP server. To restate this: use of this type of URL is prohibited to anonymous IMAP sessions, i.e., any URLFETCH command containing this type of URL issued in an anonymous session MUST return NIL in the URLFETCH response.
The "anonymous" <access> identifier indicates that use of this URL is not restricted by session authorization identity; that is, any IMAP session in authenticated or selected state (as defined in [IMAP4]), including anonymous sessions, may issue a URLFETCH [URLAUTH] using this URL.
The authorization token is represented as an ASCII-encoded hexadecimal string, which is used to authorize the URL. The length and the calculation of the authorization token depend upon the mechanism used, but in all cases, the authorization token is at least 128 bits (and therefore at least 32 hexadecimal digits).
Relative IMAP URLs are permitted and are resolved according to the rules defined in [URI-GEN]. In particular, in IMAP URLs parameters (such as ";uid=" or ";section=") are treated as part of the normal path with respect to relative URL resolution.
[URI-GEN] defines four forms of relative URLs: <inetwork-path>, <iabsolute-path>, <irelative-path>, and <ipath-empty>. Their syntax is defined in Section 11.
A relative reference that begins with two slash characters is termed a network-path reference (<inetwork-path>); such references are rarely used, because in most cases they can be replaced with an equivalent absolute URL. A relative reference that begins with a single slash character is termed an absolute-path reference (<iabsolute-path>; see also Section 7.1). A relative reference that does not begin with a slash character is termed a relative-path
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reference (<irelative-path>; see also Section 7.2). The final form is <ipath-empty>, which is "same-document reference" (see Section 4.4 of [URI-GEN]).
The following observations about relative URLs are important:
The <iauth> grammar element (which is a part of <iuserinfo>, which is, in turn, a part of <iserver>; see Section 3) is considered part of the user name for purposes of resolving relative IMAP URLs. This means that unless a new user name/server specification is included in the relative URL, the authentication mechanism is inherited from the base IMAP URL.
URLs always use "/" as the hierarchy delimiter for the purpose of resolving paths in relative URLs. IMAP4 permits the use of any hierarchy delimiter in mailbox names. For this reason, relative mailbox paths will only work if the mailbox uses "/" as the hierarchy delimiter. Relative URLs may be used on mailboxes that use other delimiters, but in that case, the entire mailbox name MUST be specified in the relative URL or inherited as a whole from the base URL.
If an IMAP server allows for mailbox names starting with "./" or "../", ending with "/." or "/..", or containing sequences "/../" or "/./", then such mailbox names MUST be percent-encoded as described in [URI-GEN]. Otherwise, they would be misinterpreted as dot- segments (see Section 3.3 of [URI-GEN]), which are processed specially during the relative path resolution process.
A relative reference that begins with a single slash character is termed an absolute-path reference (see Section 4.2 of [URI-GEN]). If an IMAP server permits mailbox names with a leading "/", then the leading "/" MUST be percent-encoded as described in [URI-GEN]. Otherwise, the produced absolute-path reference URI will be misinterpreted as a network-path reference [URI-GEN] described by the <inetwork-path> non-terminal.
A relative reference that does not begin with a slash character is termed a relative-path reference [URI-GEN]. Implementations MUST NOT generate or accept relative-path IMAP references.
See also Section 4.2 of [URI-GEN] for restrictions on relative-path references.
IMAP4, Section 5.1.3 [IMAP4] includes a convention for encoding non- US-ASCII characters in IMAP mailbox names. Because this convention is private to IMAP, it is necessary to convert IMAP's encoding to one that can be more easily interpreted by a URL display program. For this reason, IMAP's modified UTF-7 encoding for mailboxes MUST be converted to UTF-8 [UTF-8]. Since 8-bit octets are not permitted in URLs, the UTF-8 octets are percent-encoded as required by the URL specification [URI-GEN], Section 2.1. Sample code is included in Appendix A to demonstrate this conversion.
IMAP user names are UTF-8 strings and MUST be percent-encoded as required by the URL specification [URI-GEN], Section 2.1.
Also note that IMAP SEARCH criteria can contain non-US-ASCII characters. 8-bit octets in those strings MUST be percent-encoded as required by the URL specification [URI-GEN], Section 2.1.
The following examples demonstrate how an IMAP4 client program might translate various IMAP4 URLs into a series of IMAP4 commands. Commands sent from the client to the server are prefixed with "C:", and responses sent from the server to the client are prefixed with "S:".
<connect to psicorp.example.org, port 143> S: * OK [CAPABILITY IMAP4rev1 STARTTLS AUTH=CRAM-MD5] Welcome C: A001 LOGIN ANONYMOUS bester@psycop.psicorp.example.org C: A002 SELECT ~peter/&ZeVnLIqe-/&U,BTFw- <commands the client uses for viewing the contents of the mailbox>
<connect to minbari.example.org, port 143> S: * OK Greetings C: A000 CAPABILITY S: * CAPABILITY IMAP4rev1 STARTTLS AUTH=GSSAPI S: A000 OK C: A001 AUTHENTICATE GSSAPI <authentication exchange> C: A002 SELECT gray-council C: A003 UID FETCH 20 BODY.PEEK[1.2]
If the following relative URL is located in that body part:
<;section=1.4>
this could result in the following client commands:
C: A004 UID FETCH 20 (BODY.PEEK[1.2.MIME] BODY.PEEK[1.MIME] BODY.PEEK[HEADER.FIELDS (Content-Location)]) <Client looks for Content-Location headers in result. If no such headers, then it does the following> C: A005 UID FETCH 20 BODY.PEEK[1.4]
<connect to minbari.example.org, port 143> S: * OK Welcome C: A001 CAPABILITY S: * CAPABILITY IMAP4rev1 AUTH=DIGEST-MD5 S: A001 OK C: A002 AUTHENTICATE DIGEST-MD5 <authentication exchange> S: A002 OK user lennier authenticated C: A003 SELECT "gray council" ... C: A004 SEARCH SUBJECT shadows S: * SEARCH 8 10 13 14 15 16 S: A004 OK SEARCH completed C: A005 FETCH 8,10,13:16 ALL ...
In the example above, the client has implementation-dependent choices. The authentication mechanism could be anything, including PREAUTH. The final FETCH command could fetch more or less information about the messages, depending on what it wishes to display to the user.
That is, both of them reference the mailbox "foo" located on the IMAP server described by the corresponding Base URI.
The following relative-path reference
<;UID=20>
references a message with UID in the mailbox specified by the Base URI.
The following edge case example demonstrates that the ;UIDVALIDITY= modifier is a part of the mailbox name as far as relative URI resolution is concerned:
<..;UIDVALIDITY=385759045/;UID=20>
In this example, ".." is not a dot-segment [URI-GEN].
Security considerations discussed in the IMAP specification [IMAP4] and the URI specification [URI-GEN] are relevant. Security considerations related to authenticated URLs are discussed in Section 3.2 of this document.
Many email clients store the plaintext password for later use after logging into an IMAP server. Such clients MUST NOT use a stored password in response to an IMAP URL without explicit permission from the user to supply that password to the specified host name.
Clients resolving IMAP URLs that wish to achieve data confidentiality and/or integrity SHOULD use the STARTTLS command (if supported by the
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server) before starting authentication, or use a SASL mechanism, such as GSSAPI, that provides a confidentiality security layer.
10.1. Security Consideration Specific to URLAUTH Authorized URL
The "user+<userid>" <access> identifier limits resolution of that URL to a particular userid, whereas the "submit+<userid>" <access> identifier is more general and simply requires that the session be authorized by a user that has been granted a "submit" role within the authentication system. Use of either of these mechanisms limits the scope of the URL. An attacker who cannot authenticate using the appropriate credentials cannot make use of the URL.
The "authuser" and "anonymous" <access> identifiers do not have this level of protection. These access identifiers are primarily useful for public export of data from an IMAP server, without requiring that it be copied to a web or anonymous FTP server.
The decision to use the "authuser" <access> identifier should be made with caution. An "authuser" <access> identifier can be used by any authorized user of the IMAP server; therefore, use of this access identifier should be limited to content that may be disclosed to any authorized user of the IMAP server.
The decision to use the "anonymous" <access> identifier should be made with extreme caution. An "anonymous" <access> identifier can be used by anyone; therefore, use of this access identifier should be limited to content that may be disclosed to anyone.
Formal syntax is defined using ABNF [ABNF], extending the ABNF rules in Section 9 of [IMAP4]. Elements not defined here can be found in [ABNF], [IMAP4], [IMAPABNF], or [URI-GEN]. Strings are not case sensitive, and free insertion of linear white space is not permitted.
sub-delims-sh = "!" / "$" / "'" / "(" / ")" / "*" / "+" / "," ;; Same as [URI-GEN] sub-delims, ;; but without ";", "&" and "=".
uchar = unreserved / sub-delims-sh / pct-encoded
achar = uchar / "&" / "=" ;; Same as [URI-GEN] 'unreserved / sub-delims / ;; pct-encoded', but ";" is disallowed.
bchar = achar / ":" / "@" / "/"
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enc-auth-type = 1*achar ; %-encoded version of [IMAP4] "auth-type"
enc-mailbox = 1*bchar ; %-encoded version of [IMAP4] "mailbox"
enc-search = 1*bchar ; %-encoded version of [IMAPABNF] ; "search-program". Note that IMAP4 ; literals may not be used in ; a "search-program", i.e., only ; quoted or non-synchronizing ; literals (if the server supports ; LITERAL+ [LITERAL+]) are allowed.
enc-section = 1*bchar ; %-encoded version of [IMAP4] "section-spec"
enc-user = 1*achar ; %-encoded version of [IMAP4] authorization ; identity or "userid".
iserver = [iuserinfo "@"] host [ ":" port ] ; This is the same as "authority" defined ; in [URI-GEN]. See [URI-GEN] for "host" ; and "port" definitions.
iuid = "/" iuid-only
iuid-only = ";UID=" nz-number ; See [IMAP4] for "nz-number" definition
iuserinfo = enc-user [iauth] / [enc-user] iauth ; conforms to the generic syntax of ; "userinfo" as defined in [URI-GEN].
partial-range = number ["." nz-number] ; partial FETCH. The first number is ; the offset of the first byte, ; the second number is the length of ; the fragment.
uidvalidity = ";UIDVALIDITY=" nz-number ; See [IMAP4] for "nz-number" definition
The imap: URI scheme is used to designate IMAP servers, mailboxes, messages, MIME bodies [MIME] and their parts, and search programs on Internet hosts accessible using the IMAP protocol.
Applications/protocols that use this URI scheme name:
The imap: URI is intended to be used by applications that might need access to an IMAP mailstore. Such applications may include (but are not limited to) IMAP-capable web browsers; IMAP clients that wish to access a mailbox, message, or edit a message on the server using [CATENATE]; [SUBMIT] clients and servers that are requested to assemble a complete message on submission using [BURL].
Interoperability considerations:
A widely deployed IMAP client Netscape Mail (and possibly Mozilla/Thunderbird/Seamonkey) uses a different imap: scheme internally.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[IMAP4] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION 4rev1", RFC 3501, March 2003.
[IMAPABNF] Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4 ABNF", RFC 4466, April 2006.
[ABNF] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996.
[URI-GEN] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[NAMESPACE] Gahrns, M. and C. Newman, "IMAP4 Namespace", RFC 2342, May 1998.
[LITERAL+] Myers, J., "IMAP4 non-synchronizing literals", RFC 2088, January 1997.
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[ANONYMOUS] Zeilenga, K., "Anonymous Simple Authentication and Security Layer (SASL) Mechanism", RFC 4505, June 2006.
[DATETIME] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, July 2002.
[URLAUTH] Crispin, M., "Internet Message Access Protocol (IMAP) - URLAUTH Extension", RFC 4467, May 2006.
Here is sample C source code to convert between URL paths and IMAP mailbox names, taking into account mapping between IMAP's modified UTF-7 [IMAP4] and hex-encoded UTF-8, which is more appropriate for URLs. This code has not been rigorously tested nor does it necessarily behave reasonably with invalid input, but it should serve as a useful example. This code just converts the mailbox portion of the URL and does not deal with parameters, query, or server components of the URL.
/* Copyright (C) The IETF Trust (2007). This version of sample C code is part of RFC XXXX; see the RFC itself for full legal notices.
Regarding this sample C code (or any portion of it), the authors make no guarantees and are not responsible for any damage resulting from its use. The authors grant irrevocable permission to anyone to use, modify, and distribute it in any way that does not diminish the rights of anyone else to use, modify, and distribute it, provided that redistributed derivative works do not contain misleading author or version information.
Derivative works need not be licensed under similar terms. */
/* "gen-delims" excluding "/" but including "%" */ #define GENERAL_DELIMS_NO_SLASH ":?#[]@" "%"
/* "gen-delims" (excluding "/", but including "%") plus subset of "sub-delims" */ #define GENERAL_UNSAFE_NO_SLASH GENERAL_DELIMS_NO_SLASH ";&=+" #define OTHER_UNSAFE " \"<>\\^`{|}"
/* Convert an IMAP mailbox to a URL path * dst needs to have roughly 4 times the storage space of src * Hex encoding can triple the size of the input * UTF-7 can be slightly denser than UTF-8 * (worst case: 8 octets UTF-7 becomes 9 octets UTF-8) */ void MailboxToURL(char *dst, char *src) { unsigned char c, i, bitcount; unsigned long ucs4, utf16, bitbuf; unsigned char base64[256], utf8[6];
/* initialize modified base64 decoding table */
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memset(base64, UNDEFINED, sizeof (base64)); for (i = 0; i < sizeof (base64chars); ++i) { base64[(int) base64chars[i]] = i; }
/* loop until end of string */ while (*src != '\0') { c = *src++; /* deal with literal characters and &- */ if (c != '&' || *src == '-') { /* NB: There are no "URL safe" characters after the '~' */ if (c < ' ' || c > '~' || strchr(mailbox_url_unsafe, c) != NULL) { /* hex encode if necessary */ dst[0] = '%'; dst[1] = hex[c >> 4]; dst[2] = hex[c & 0x0f]; dst += 3; } else { /* encode literally */ *dst++ = c; } /* skip over the '-' if this is an &- sequence */ if (c == '&') ++src;
/* Convert hex coded UTF-8 URL path to modified UTF-7 IMAP mailbox * dst should be about twice the length of src to deal with non-hex * coded URLs */ int URLtoMailbox(char *dst, char *src) { unsigned int utf8pos = 0; unsigned int utf8total, i, c, utf7mode, bitstogo, utf16flag; unsigned long ucs4 = 0, bitbuf = 0;
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utf7mode = 0; /* is the output UTF7 currently in base64 mode? */ utf8total = 0; /* how many octets is the current input UTF-8 char; 0 == between characters */ bitstogo = 0; /* bits that need to be encoded into base64; if bitstogo != 0 then utf7mode == 1 */ while ((c = (unsigned char)*src) != '\0') { ++src; /* undo hex-encoding */ if (c == '%' && src[0] != '\0' && src[1] != '\0') { c = HEXCHAR(src[0]); i = HEXCHAR(src[1]); if (c == XX || i == XX) { return 0; } else { c = (char)((c << 4) | i); } src += 2; } /* normal character? */ if (c >= ' ' && c <= '~') { /* switch out of UTF-7 mode */ if (utf7mode) { if (bitstogo) { *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F]; } *dst++ = '-'; utf7mode = 0; bitstogo = bitbuf = 0; } *dst++ = c; /* encode '&' as '&-' */ if (c == '&') { *dst++ = '-'; } continue; } /* switch to UTF-7 mode */ if (!utf7mode) { *dst++ = '&'; utf7mode = 1; } /* Encode US-ASCII characters as themselves */ if (c < 0x80) { ucs4 = c; utf8total = 1; } else if (utf8total) { /* this is a subsequent octet of a multi-octet character */ /* save UTF8 bits into UCS4 */
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ucs4 = (ucs4 << 6) | (c & 0x3FUL); if (++utf8pos < utf8total) { continue; } } else { /* this is the first octet of a multi-octet character */ utf8pos = 1; if (c < 0xE0) { utf8total = 2; ucs4 = c & 0x1F; } else if (c < 0xF0) { utf8total = 3; ucs4 = c & 0x0F; } else { /* NOTE: can't convert UTF8 sequences longer than 4 */ utf8total = 4; ucs4 = c & 0x03; } continue; } /* Finished with UTF-8 character. Make sure it isn't an overlong sequence. If it is, return failure. */ if ((ucs4 < 0x80 && utf8total > 1) || (ucs4 < 0x0800 && utf8total > 2) || (ucs4 < 0x00010000 && utf8total > 3) || (ucs4 < 0x00200000 && utf8total > 4) || (ucs4 < 0x04000000 && utf8total > 5) || (ucs4 < 0x80000000 && utf8total > 6)) { return 0; } /* loop to split ucs4 into two utf16 chars if necessary */ utf8total = 0; do { if (ucs4 >= UTF16BASE) { ucs4 -= UTF16BASE; bitbuf = (bitbuf << 16) | ((ucs4 >> UTF16SHIFT) + UTF16HIGHSTART); ucs4 = (ucs4 & UTF16MASK) + UTF16LOSTART; utf16flag = 1; } else { bitbuf = (bitbuf << 16) | ucs4; utf16flag = 0; } bitstogo += 16; /* spew out base64 */ while (bitstogo >= 6) { bitstogo -= 6; *dst++ = base64chars[(bitstogo ? (bitbuf >> bitstogo)
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: bitbuf) & 0x3F]; } } while (utf16flag); } /* if in UTF-7 mode, finish in ASCII */ if (utf7mode) { if (bitstogo) { *dst++ = base64chars[(bitbuf << (6 - bitstogo)) & 0x3F]; } *dst++ = '-'; } /* tie off string */ *dst = '\0'; return 1; }
Updated boilerplate, list of editor's, etc. Updated references. Updated ABNF not to use _, to use SP instead of SPACE, etc. Updated example domains to use example.org. Fixed ABNF error in "imessagelist" non-terminal. Updated ABNF, due to changes in RFC 3501, RFC 4466, and RFC 3986. Renamed "iuserauth" non-terminal to <iuserinfo>. Clarified that the userinfo component describes both authorization identity and mailbox naming scope. Allow for non-synchronizing literals in "enc-search". Added "ipartial" specifier that denotes a partial FETCH. Moved URLAUTH text from RFC 4467 to this document. Updated ABNF for the whole server to allow missing trailing "/" (e.g., "imap://imap.example.com" is now valid and is the same as "imap://imap.example.com/"). Clarified how relative-path references are constructed. Added more examples demonstrating relative-path references. Added rules for relative URLs and restructured ABNF as the result. Removed text on use of relative URLs in MHTML. Added examples demonstrating security considerations when resolving URLs. Recommend usage of STARTTLS/SASL security layer to protect confidential data. Removed some advices about connection reuse that were incorrect. Removed URLs referencing a list of mailboxes, as this feature hasn't seen any deployments. Clarified that user name "anonymous" is case-insensitive.
Text describing URLAUTH was lifted from [URLAUTH] by Mark Crispin.
Stephane H. Maes contributed some ideas to this document; he also co-edited early versions of this document.
The editors would like to thank Mark Crispin, Ken Murchison, Ted Hardie, Zoltan Ordogh, Dave Cridland, Kjetil Torgrim Homme, Lisa Dusseault, Spencer Dawkins, Filip Navara, Shawn M. Emery, Sam Hartman, Russ Housley, and Lars Eggert for the time they devoted to reviewing this document and/or for the comments received.
Authors' Addresses
Chris Newman (Author/Editor) Sun Microsystems 3401 Centrelake Dr., Suite 410 Ontario, CA 91761 EMail: chris.newman@sun.com
Alexey Melnikov (Editor) Isode Limited 5 Castle Business Village 36 Station Road Hampton, Middlesex TW12 2BX, UK EMail: Alexey.Melnikov@isode.com URI: http://www.melnikov.ca/
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